1. Field of Embodiments
This invention relates to the field of radiation detection systems for scanning of persons, luggage, parcels, vehicles and containers for the presence of illegal nuclear and radioactive materials. Specifically, this invention is related to methods and techniques of minimizing or removing the unintended interference from pulsed X-ray radiation generated by high-energy radiography systems, operating in proximity of passive radiation detection systems.
2. Summary of Related Art
Currently, second-generation high-energy radiography systems are being more frequently deployed alongside passive radiation detection systems, in order to interdict illegal transportation of contraband and other dangerous goods including nuclear and radioactive materials. It is well known that X-rays from nearby pulsed high-energy imaging systems can interfere with the radiation portal monitors (RPM) used to perform passive radiation monitoring. Such interference results in an increased count rate measured by the RPM which could lead to, for example, false alarms on vehicles with no radioactive materials present, when the additional counts increase RPM's measured signal above the set alarm threshold. Increased count rate could also raise the measured background above the actual background level when the X-ray interference occurs during RPM background acquisition, leading to a loss of sensitivity and decreased minimum detectable activity. Second order effects of the X-ray interference may also include: short-term gain shift, spectrum distortion, degraded energy resolution, incomplete pile-up rejection and decreased live time, all with negative effects on the RPM's detection performance.
Various solutions have been adopted in order to mitigate the effects of pulsed X-rays radiation from pulsed X-rays radiography systems on the nearby radiation portal monitors.
A first class of such solutions is using distance, time or shielding in order to minimize the X-rays detected by the RPM. For example, placing the radiography system 300-500 feet away from all the RPMs operating within one venue, could reduce the X-rays to a level that allows each RPM to operate virtually free of X-ray interference. The same effect could be achieved by surrounding radiography system with adequate shielding walls or imposing an exclusion mechanism between the time intervals when RPMs are operating and the times when radiography system(s) are allowed to operate. All three approaches described above, used alone or in combination, are simple, and, in some isolated cases, cost-effective ways to address the X-rays interference. They do not rely on any particular characteristics of radiography system and RPM. Unfortunately, practical constraints imposed by limited real-estate combined with prohibitive high cost of shielding walls, high volume of traffic, and very large number of RPMs and radiography system that have to operate in close proximity, reduce the number of sites where these solutions can be applied, to a very few isolated cases, while the vast majority remains with X-ray interference problem unsolved.
Another widely adopted class of solutions, called hardware or predictive blanking uses a modified RPM such that it is able to discard its input signals during a small time interval (typical less than 20 μs) while the X-rays pulse is generated by the radiography system. As X-rays pulse repetition rate ranges typically from 100 to 500 Hz, RPM will be blanked, in the worst case 20 μs every 2 ms, resulting in an equivalent 1% dead time. This additional dead time imposed by blanking, being less than 1% should, in theory, have negligible effect on the RPM's detection performance. The method requires a hardware synchronization signal generated by the radiography system. This logic signal known as blanking sync, becomes active a known time interval before the X-rays pulse is generated. While the time delay between blanking sync and X-ray pulse is approximately constant for a given system and operating mode, it has a wide range of values for different manufacturers and/or operating modes. RPM receives blanking sync from the X-ray imaging system, and generates an internal detector gating signal called blanking pulse. The blanking pulse is delayed with respect of the external blanking sync by a set amount known as blanking delay, and it is active for a set time interval known as blanking time. The values for blanking delay and blanking time are set such that the X-ray pulse falls always inside the blanking pulse with sufficient margins on both left and right sides. These tolerances are required to compensate for inherent timing jitter present in the signals involved from both participating systems: X-ray imaging and RPM. Blanking pulse is then distributed to each gamma and each neutron detector within the same RPM. For some implementations of this method, blanking delay and blanking time may have different values for each individual gamma and neutron detector within the same RPM. The RPM implementing this method is capable of disabling its gamma and neutron detectors while internal blanking pulse is active with the end effect of removing all pulses while the X-rays are being generated. Hardware blanking solution works relatively well when there is a single pulsed X-ray source, operating at a single energy with only one RPM needed to be blanked. The major disadvantage of this method is related to the requirement that there is a wired physical connection between the X-ray imaging system and RPM, therefore making it practically impossible to implement in the case of mobile X-rays imagining systems or when the blanking sync has to be distributed to a large number of RPMs. The solution becomes extremely difficult to implement when there are multiple radiography systems with different operating modes, (single energy, dual energy e.g., interleaved higher and lower energy pulses), and mobile systems, operating in close proximity of a large number of RPM systems.
There is a need in the art for an X-rays interference solution that allows for multiple and/or mobile radiography systems, including dual energy mode systems, operating in proximity to multiple radiation portal monitors that eliminates pulsed X-rays interference and renders the operation and specifically detection and nuclide classification performance, virtually unaffected by the pulsed X-rays generated by high-energy imaging systems.